Recs.
Updated
Tornado is a scalable, non-blocking web server and web application framework. It was developed for use by FriendFeed, which was acquired by Facebook in 2009 and Tornado was open-sourced soon after.
Specs
Pros
Pro Light, async, fast
Light weight, asynchronous, fast, it's better to use Tornado with other frameworks.
Pro Lightweight async server supporting websockets
Tornado is a Python web framework and asynchronous networking library, originally developed at FriendFeed. By using non-blocking network I/O, Tornado can scale to tens of thousands of open connections, making it ideal for long polling, WebSockets, and other applications that require a long-lived connection to each user.
Cons
Con It's usually useful only when building real-time web applications
Because it's basically a framework for building real-time applications it has great support for things like WebScokets and uses non-blocking I/O. But it lacks in features that are used to build general-purpose websites like blogs, e-commerce sites, web portals etc...
For example it lacks ORM integration (at least out of the box) and it doesn't have a very powerful template engine.